Rethinking the Reactivity of Nanoscale Metal Oxides

金属氧化物材料广泛存在，从环境中的岩石，到燃料电池的组件，再到防晒霜的成分。这些材料越来越多地用作非常小的颗粒（纳米颗粒-直径为十亿分之一米的颗粒）。纳米粒子的行为和用途往往在添加或去除几个原子后发生变化。 在这个研究项目中，耶鲁大学的Mayer教授的团队正在研究氢（H）和氧（O）原子如何以及为什么在金属氧化物纳米颗粒上移动。氢和氧无处不在，因为它们存在于水（H2O）、空气（O2）和碳氢化合物燃料（如汽油）中。了解金属氧化物如何使用或提供H和O可能会带来新的想法，也许还会开发新技术。青年科学家的发展和更广泛社区的参与是该项目相互交织的目标。计划举办公开讲座，并正在制定一个简短的课堂方案。 Mayer教授和他的团队还准备了为期一周的研讨会，并提供了为期一个月的实习机会。这些活动强调动手和发现为基础的活动，以吸引不同群体的高中生，特别是那些目前在STEM领域代表性不足的群体。 金属氧化物纳米晶体（MOx NC）的氧化还原（氧化/还原）反应通常被描述为外层电子转移过程。然而，越来越多的证据表明，这些界面反应中的许多（如果不是大多数）实际上是内球过程，涉及O-H和M-O化学键的形成和断裂。涉及氧化物材料的O-H键的反应可以被描述为质子耦合电子转移或氢原子转移。质子耦合电子转移到MOx NC涉及氧化物质子化为氢氧化物和电子添加到带态或陷阱态。正在研究各种MOx NC的内球氧化还原反应性，包括n型半导体TiO 2和CeO 2-x，p型NiO和金属RuO 2。氧原子转移反应也在探索中。这些反应尚未很好地建立NC/溶液界面。实验采用分子方法，利用溶液化学和材料研究的工具。例如，纳米晶体与已知经历氢原子转移和氧原子转移的分子基底反应。氢原子转移反应提供了热化学信息，这是组装成一个新的氢原子亲和尺度的纳米晶体。反应的动力学与热化学驱动力相关，以检查速率与驱动力的比例关系。氢原子转移和氧原子转移反应的平行研究显示了这些基本反应类型之间的联系。这些对单个反应步骤的基础研究为纳米材料反应性建立了一个潜在的变革性新范式，植根于热化学和动力学。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Metal-oxide materials are widespread, from rocks in the environment, to components of fuel cells, to ingredients in sunscreens. These materials are increasingly used as very small particles (nanoparticles - particles that are a billionth of a meter in diameter). The behavior and use of nanoparticles often change after adding or removing just a few atoms. In this research project, Professor Mayer's group at Yale University is studying how and why hydrogen (H) and oxygen (O) atoms move onto and out of metal oxide nanoparticles. Hydrogen and oxygen are ubiquitous, as they are in water (H2O), in air (O2) and in hydrocarbon fuels like gasoline. Understanding how metal oxides use or provide H and O could lead to new ideas and perhaps the development of new technologies. The development of young scientists and the engagement of the broader community are intertwined goals of this project. Public lectures are planned and a short classroom program is being developed. Professor Mayer and his team also prepare week-long workshops and month-long internships are being offered. These activities emphasize hands-on and discovery-based activities to engage a diverse group of high school students, especially those from groups currently under-represented in STEM fields. Redox (oxidation/reduction) reactions of metal oxide nanocrystals (MOx NCs) are typically described as outer-sphere electron transfer processes. However, there is increasing evidence that many, if not most, of these interfacial reactions are actually inner-sphere processes, involving the making and breaking of O-H and M-O chemical bonds. Reactions involving O-H bonds of oxide materials can be described as proton-coupled electron transfer or hydrogen atom transfer. Proton-coupled electron transfer to a MOx NC involves protonation of an oxide to a hydroxide and addition of an electron to a band state or trap state. The inner-sphere redox reactivity of a variety of MOx NCs is being studied, including the n-type semiconductors TiO2 and CeO2-x, p-type NiO and metallic RuO2. Oxygen atom transfer reactions are also being explored. These reactions are not yet well established for NC/solution interfaces. The experiments take a molecular approach, utilizing tools from both solution chemistry and materials research. For example, the nanocrystals are reacted with molecular substrates known to undergo hydrogen atom transfer and oxygen atom transfer. The hydrogen atom transfer reactions are providing thermochemical information, which is assembled into a novel hydrogen-atom affinity scale for nanocrystals. The kinetics of the reactions are correlated with the thermochemical driving forces to examine rate vs. driving force scaling relationships. Parallel studies of hydrogen atom transfer and oxygen atom transfer reactions show the connections between these basic reaction types. These fundamental studies of individual reaction steps build a potentially transformative new paradigm for nanomaterial reactivity, rooted in thermochemistry and kinetics.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.